A gene is a unit of heredity that confers some trait or function on the organism. Most genes are thought to be essential to development and normal functioning. Genes are often a primary determinant of interpersonal differences; for example, they determine whether you have blue or brown eyes. A disrupted or mutated gene can cause serious, even fatal problems. Genes are composed of DNA and found in the chromosomes in the nucleus of a cell. At present, we are on the verge of identifying all the human genes due to the efforts of the genome projects.